Projection display devices are known as display devices for the user to enjoy video and images on a large screen. Projection display devices of the background art mainly achieve an enlarged-scale display according to the following procedure:
White color from a light source is separated into red light, green light, and blue light by a color separating means, and the separated monochromatic lights are modulated by a light modulator such as a liquid crystal device or a DMD.
The modulated images in the respective hues are combined to generate a color image by a color combining means such as a cross dichroic prism.
The generated color image is displayed at an enlarged scale on a screen or the like by a projection optical system such as a projection lens or the like.
Heretofore, discharge lamps such as high-pressure mercury lamps and metal halide lamps have been used as the light source. In recent years, the use of semiconductor light-emitting elements such as LEDs and semiconductor lasers as the light source has been proposed, and they have actually been used.
These light-emitting elements are advantageous in that they do not emit heat rays and ultraviolet rays compared with the discharge lamps, can easily control energization for light emitting elements, have a high response speed, do not run the risk of rupture, and have a long service life.
In addition, since the light-emitting elements are capable of emitting monochromatic lights in three primaries for producing color video images from red, green, and blue colors, and are also free from the need for color separation, they are appropriate as a light source for projection display devices.
A light-emitting element is mechanically and electrically connected to a board or a lead frame by silver paste, bonding wires, and stud bumps, and sealed and protected by a sealing material such as epoxy resin, silicone gel, or the like.
Since the sealing material has a refractive index of about 1.5, it is effective to increase the efficiency with which to extract light therefrom because of a reduction in the total reflection from a light-emitting device interface of light that is emitted from a light-emitting layer of the light-emitting element in the sealing material.
Light-emitting elements use different light-emitting materials depending on lights to be emitted in different colors. Generally, GaN-base compound semiconductors are widely used to emit light in a wavelength range from ultraviolet to green spectrum, and AlGaInP-base compound semiconductors, referred to as a quaternary material, and GaAs-base compound semiconductors are widely used to emit light in a wavelength range from yellow to red spectrum.
Light-emitting elements emit a lot of heat as well as light. When a large current is supplied to a light-emitting element in order to increase the amount of light emitted thereby, its temperature rises. The light emission efficiency of a light-emitting element is temperature-dependent. Generally, as the junction temperature of a light-emitting element rises, its light emission efficiency decreases. The temperature dependency differs with materials. For example, a red light-emitting element has a large temperature dependency, a green light-emitting element has a small temperature dependency, and a green light-emitting element is almost temperature-independent.
Since different light-emitting materials emit lights in different colors as described above, the degree to which the amount of light emitted at a high temperature is lowered also varies depending on the color of the emitted light. Therefore, when the environmental temperature of a projection display device varies, since the junction temperature of the light-emitting elements also varies, the ratio of the amounts of color lights varies. In other words, it is difficult to keep a desired white balance.
Light-emitting elements further have a problem in that the degree to which the amount of light is lowered due to aging is different depending on the color of the emitted light. Those light-emitting elements which emit lights in shorter wavelengths such as ultraviolet and blue spectrum have their sealing materials more liable to turn yellow than those light-emitting elements which emit lights in longer wavelengths such as red spectrum. Since the transmittance is reduced by yellowing, the amount of light passing through the sealing material is lowered.
Inasmuch as the degree to which the amount of light that is lowered due to long-term use is different depending on the color of the emitted light, maintaining projected images that have the desired white balance for producing white color, that is required for projection display devices, is difficult to achieve over a long period of time.
FIG. 1 is a schematic diagram showing an arrangement of the optical system of a projection display device disclosed in Patent document 1 (JP2007-65012A).
As shown in FIG. 1, optical system 1051 of projection display device 1001 includes R (red) color light source 1010R, G (green) color light source 1010G, B (blue) color light source 1010B, color combining means 1011, light modulator 1014, projection optical system 1016, and photodetector 1017.
Light sources 1010R, 1010G, 1010B and light modulator 1014 are controlled by controller 1100. R color light source 1010R, G color light source 1010G, and B color light source 1010B that incorporate light-emitting elements therein emit monochromatic lights, which are combined by color combining means 1011 to travel along one light path to polarizer 1013. Although not shown, a polarization unifying means including a polarization beam splitter and a λ/2 phase difference plate is inserted in front of polarizer 1013.
The polarization unifying means applies only a linearly polarized light in one polarized direction through polarizer 1013 to light modulator 1014. In this example, the optical system employs a single-plate light modulator, and hence incorporates an FSC (Field Sequential Color) display system wherein lights in three primaries of red, blue, and green are successively emitted and modulated by light modulator 1014, after which the modulated lights are combined over time to produce a full-color image. Light modulator 1014 controls the direction of polarization of the linearly polarized light applied thereto, depending on the image to be displayed, and the image is projected through detecting assembly 1102 and projection optical system 1016 onto a screen or the like, not shown.
Polarizer 1013 is disposed on a light entrance side of light modulator 1014, and polarization splitting means 1015 is disposed on a light exist side of light modulator 1014. Light modulator 1014 comprises a single liquid crystal device. Polarization splitting means 1015, which comprises a polarization beam splitter or the like, has polarization splitting surface 1030 in the form of a dichroic film that is inclined 45° to the applied light. Polarization splitting means 1015 has a function to pass a linearly polarized light beam in one polarized direction and reflect another linearly polarized light beam in a polarized direction perpendicular thereto.
In FIG. 1, polarization splitting means 1015 passes a linearly polarized light beam in a polarized direction parallel to the sheet of FIG. 1 (hereinafter referred to as “P-polarized light”) and reflects a linearly polarized light beam in a polarized direction perpendicular to the sheet of FIG. 1 (hereinafter referred to as “S-polarized light”). As indicated by the broken-line arrow, projection optical system 1016 is disposed on the light path of the P-polarized light beam which represents transmitted light 1020A having passed through polarization splitting means 1015. Projection optical system 1016 projects the light modulated by light modulator 1014 onto a screen or the like, not shown. As indicated by the solid-line arrow, the S-polarized light beam which represents reflected light 1020B reflected by polarization splitting surface 1030 of polarization splitting means 1015 is applied to photodetector 1017. Photodetector 1017 may comprise a photodiode, a phototransistor, or the like for converting the intensity of light into an electric quantity. Polarization splitting means 1015 and photodetector 1017 function as detector 1102. Controller 1100 has a function to energize R color light source 1010R, G color light source 1010G, and B color light source 1010B, and a function to control light modulator 1014 and photodetector 1017. When the illuminance levels of the R, G, B color light sources are detected and supplied to controller 1100 via a feed-back loop, controller 1100 controls the electric power levels of the color light sources in order to keep the white balance at a desired value.
The display device disclosed in JP2007-65012A is problematic in that though it can deal with a reduction in light emission efficiency due to aging of the light-emitting elements, it fails to maintain white balance when the environmental temperature varies since the temperature dependency of the light-emitting elements is not taken into account.
Patent document 1: JP2007-65012A